The present disclosure relates to, for example, a current resonance type switching power supply device which outputs a stable DC voltage, a switching power supply control method, and an electronic apparatus.
As one switching power supply device, a current resonance type switching power supply device is known. The current resonance type switching power supply device has the following advantages.
1. Zero volt switching (ZVS) is performed in the entire load range of no load to a maximum load
2. A current at the time of turning off of a primary side switching element can be decreased
3. Zero current switching (ZCS) of a secondary side rectifying element is performed
4. Since it is possible to perform a voltage boosting operation without lowering efficiency at a normal time, the current resonance type switching power supply device is suitable for a power supply which is necessary to secure a retention time
The current resonance type switching power supply device includes a switching circuit which has two switching elements connected in series. The two switching elements are alternately turned on and off. A transformer is connected to the switching elements, and an AC voltage, which is induced in the secondary side of the transformer, is rectified and becomes an output voltage.
In a case where a power conversion efficiency of the current resonance type switching power supply device is improved, it is necessary for a power loss of the transformer to be considered. The power loss occurring in the transformer is a sum of a copper loss of a wire caused by a resonance current and an excitation current through the primary side, and a transfer current to the secondary side and an iron loss of a transformer core caused by excitation of the transformer. It is common for the transformer to be optimally designed such that the sum of the copper loss and the iron loss at the time of the maximum load becomes a minimum value. For example, refer to a document (R. W. Erickson, D. Maksimovic: Fundamentals of power electronics, ed. Kluwer Academic Publishers, (2nd edition) ISBN 0792372700). In addition, in the following description, the power loss is abbreviated simply to “loss”, and the power conversion efficiency is abbreviated simply to “efficiency”.
The copper loss caused by both the resonance current and the transfer current is changed in magnitude by the load. In contrast, a sum of the copper loss caused by the excitation current and the iron loss is a constant value regardless of the load. Thus, when the load is a light load, the sum of the copper loss caused by the excitation current and the iron loss increases, and when an operation point the same as that at the time of the maximum load is set, the efficiency of the current resonance type switching power supply device in the light load is lowered compared to the efficiency of a quasi-resonance circuit or the like.
it has been proposed that whether the load is a light load or a heavy load is detected, an ON period of a switching element for switching is controlled depending on whether the load is a light load or a heavy load, and the operation point is moved, and thereby the efficiency is prevented from being lowered, as disclosed in Japanese Unexamined Patent Application Publication No. 6-46561, Japanese Unexamined Patent Application Publication No. 2006-204044, and Japanese Unexamined Patent Application Publication No. 2001-333576.